How do you know if someone has WhatsApp installed?

Related questions

1. Why is WhatsApp not installing on my phone?
2. When you star a message on WhatsApp does the other person know?
3. Can anyone else see my WhatsApp messages?
4. Is GBWhatsApp safe?
5. Is WhatsApp really safe?
6. Can non contacts see WhatsApp status?
7. Why can't I install WhatsApp?
8. What is WhatsApp plus?
9. Whats is GB WhatsApp?
10. What is the difference between GBWhatsApp and WhatsApp?

Tags

1. #number
2. #select
3. #person
4. #there
5. #originally
6. #installed
7. #phone
8. #contact
9. #answered
10. #whatsapp
11. #someone